The operation of hot air furnaces to heat buildings tends to deplete the moisture content of the conditioned volume by expending warm, moist air in the combustion process and replacing it with dry air from the exterior. Moisture is also removed from the heated air by virtue of condensation on cold window and wall surfaces. Low moisture content of the heated air produces physical discomfort to the occupants of the heated building and also increases the air temperature required to produce a feeling of comfort since body moisture evaporates more readily to low humidity air and produces a body cooling effect. By way of example, it has been estimated that the degree of comfort provided a room temperature of 70.degree. F. with 50% relative humidity requires a room temperature of 76.degree. F. when the relative humidity is lowered to 10%. Extremely low relative humidity also has a deleterious effect on wood furniture within the heated building and the wooden components of the structure itself because of the resulting wood shrinkage and expansion during subsequent periods of higher relative humidity.
For these reasons humidifiers are often used with forced air heating systems to raise the relative humidity to a level that provides physical comfort and higher heating efficiency. The systems may simply depend upon the vaporization of water from a reservoir as the heated air passes over its surface or may be powered by motors or the like operated in timed relation to the furnace to provide a desired level of relative humidity. The powered devices may be provided with a humidistat to measure the relative humidity in the heated air and control the operation of the humidifier to attain a desired level of humidity.
The maximum desirable relative humidity will vary as a function of the temperature differential between the exterior surfaces of the building and the interior surfaces. For any given temperature differential in a given building structure, moisture will condense on the interior sides of exterior walls and windows if the interior relative humidity exceeds a predetermined level. For an interior temperature of 65.degree. F. in a particular structure, a relative humidity of 50% may be attained without condensation for an outside "effective temperature" of 30.degree., but a relative humidity in excess of 20% may produce undesirble condensation when the outside "effective temperature" lowers to 10.degree. F. "Effective temperature" is a function of the exterior temperature and wind velocity and is akin to what is commonly known as "wind-chill factor". The rate of heat exchange between the interior and exterior of a building structure is a function of the difference between interior temperature and this exterior "effective temperature".
In general, relative humidity may be maintained at a range limited only by comfort and heating efficiency, i.e., 40-50%, as long as the difference between the interior temperature and the effective exterior temperature is below about 35.degree. F. As the difference increases beyond that limit the maximum relative humidity within the structure which may be attained without appreciable condensation decreases.
In order to achieve the necessary regulation of interior humidity as a function of the differential between the interior temperature and exterior effective temperature, the humidistats for active humidifiers have been provided with either manual controls which allow the adjustment of relative humidity, or automatic controls employing an exterior temperature sensor, as in U.S. Pat. No. 3,840,176. The manual control systems require constant adjustment by the operator as the temperature varies. For example, the control may be set at 50% when the exterior temperature is above 30.degree. F., but during a sharp temperature drop, i.e., to 15.degree. F., it must be lowered to 20% or so. Typically, the operator will simply set the manual controls to a sufficiently low relative humidity level so that no condensation will be attained even in the coldest weather. When the exterior temperature rises the control will typically be left at this low setting until someone complains about the discomfort of the low humidity. In general, these controls have proven unsatisfactory because their proper operation requires frequent operator attention. The automatic controls employing an exterior temperature sensor are relatively expensive and difficult to install and have not attained an appreciable degree of commercial acceptance.
An alternative form of open loop control of a humidifier, which does not employ any measurement of the actual humidity in the air but rather operates the humidifier as a function of the period of operation of the furnace in order to increase the amount of moisture added to the air during colder weather, as disclosed in U.S. Pat. No. 3,464,401. However, the quantity of moisture that must be added to the air as a function of the exterior temperature in order to attain the desired relative humidity for that exterior temperature is not a straight line function and not amenable to open loop control, limiting the utility of such a system.